The invention relates to a circuit and method for driving a pixel with a pixel drive signal whose duty cycle is defined by a digital input value.
A substantial need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. In particular, wearable display devices could be used instead of the conventional displays of laptop and other portable computers, and portable Digital Versatile Disk (DVD) players. Potentially, wearable display devices can provide greater brightness, better resolution, larger apparent size, greater privacy, substantially less power consumption and longer battery life than conventional active matrix or double-scan liquid crystal-based displays. Other potential applications of wearable display devices are in personal video monitors, in video games and in virtual reality systems.
Recently, a miniature video display based on a light valve that uses a ferroelectric liquid crystal material was described in U.S. patent applications Ser. Nos. 09/070,487 and 09/070,669, assigned to the assignee of this disclosure and incorporated herein by reference. Such a miniature video display can form part of a wearable eyeglass display that can be used to display computer graphics when connected to the video output of a computer, especially a laptop computer, and can be used to display video when connected to the video output of a TV receiver, a video cassette player or a DVD player, especially a portable DVD player.
One embodiment of the light valve of such a miniature video display includes an array of 1024xc3x97768 pixels, each including a reflective electrode driven by a respective pixel driver. The pixel driver converts an analog sample derived from an analog video signal into a two-state drive signal having a duty cycle that defines the apparent brightness of the pixel. Sequentially illuminating with light of two or more different colors and setting each pixel to an apparent brightness associated with each color during the respective illumination period enables a color frame to be displayed. A similar pixel driver can be used in video displays based on other binary electro-optical transducers, such as solid-state or organic light-emitting materials, in which duty cycle of the drive signal coupled to the electro-optical transducer determines the apparent brightness of the pixel.
When the miniature video display just described is driven by a conventional analog video signal, analog samples are derived from each line of the analog video signal and are distributed via column busses to the pixel circuits in each row of the array. Recently, however, it has been proposed to use the video display just described as the viewfinder of a digital camera that generates a digital video signal. Moreover, many other video applications generate a digital video signal composed of parallel red, green and blue pixel values. To drive the above-mentioned analog video display, a digital-to-analog converter must be used to convert the digital video signal generated by the camera to an analog signal. A parallel digital-to-analog converter suitable for this purpose is described in U.S. patent application Ser. No. 09/249,600, assigned to the assignee of this application.
Using a digital-to-analog converter to convert the digital video signal to an analog signal suitable for driving the above-described analog-based miniature video display requires considerable additional circuitry and increases the power consumption of the display. Power consumption is an important consideration since the miniature video display is especially intended for use as the display for laptop computers and portable DVD players. Moreover, the analog circuitry of the miniature video display presents significant challenges when the highest picture quality is desired. Another important shortcoming is that new analog samples must be obtained from the video signal and be distributed to the pixels constituting the display after each display period, which is typically one video frame period. When the frame being displayed is relatively static, as in a computer display or the display of an electronic book, this needlessly increases the power consumption.
Miniature video displays that incorporate liquid crystal-based light valves and are indirectly driven by a digital video signal are known. In these, the digital video signal is converted into a grey scale binary-weighted, time-multiplexed, time domain binary-weighted drive signal to drive each pixel. The time domain weighting of the drive signal creates tremendous inefficiencies in the link between the converter and the display since the link is idle during the ON time of the high order bits. Including an image buffer in the display removes this inefficiency, but significantly increases the cost and power consumption of the display. The bitwise binary-weighted time domain drive signal also imposes a significant burden on the bandwidth of the liquid crystal material itself due to the switching speed necessary to display the low-order bits, which have a very short duration. Current ferroelectric liquid crystal materials do not have sufficient switching speed to display a full 24-bit (eight bits per color) color palette. This problem is further exacerbated by the desire to move process technology to lower and lower voltages, since the switching speed of ferroelectric liquid crystal material depends on the strength of the applied field, and therefore on the voltage of the drive signal.
An additional complexity is the bit reordering that must be applied to the digital video signal. Most digital video signals are composed of sets of RGB pixel values in raster scan order. Bitwise imaging requires buffering of the RGB pixel values and then reordering them into a bit-plane sequential data stream in which the lowest-order bits (for example) of all the pixels are presented first, followed by the next-but-lowest order bits of all the pixels, and so on until the highest-order bits of all the pixels are presented. Re-ordering the digital video signal requires a buffer memory that has significant bandwidth requirements, and power consumption, when the display has a high resolution.
Accordingly, what is needed is a pixel driver capable of directly receiving a pixel value constituting part of a conventional digital video signal and of generating, in response to the pixel value, a drive signal having a duty cycle that, in a monochrome display, determines the apparent brightness of the pixel and, in a color display, determines the apparent brightness of the pixel for each of two or more color components. The pixel driver should be simple, so that the pixel can be made sufficiently small to allow a high-resolution display composed of hundreds of thousands, or even millions, of pixels to be formed on a semiconductor chip having dimensions of the order of 10 mmxc3x9710 mm. The pixel driver should have low power consumption to enable it to be used in portable, battery-powered applications. Finally, when the digital video signal is relatively static, the pixel driver should be capable of operating in a mode that does not require the pixel values to be re-loaded into the pixels after each display period to reduce power consumption.
The invention provides a digital pixel driver that operates in response to an M-bit digital input value defining the apparent brightness of the pixel. The pixel driver generates a pixel drive signal having a duty cycle that sets the apparent brightness of the pixel. The pixel driver comprises a memory, a digital sequence generator and a comparator. The memory receives and stores an N-bit word that represents the digital input value. The digital sequence generator generates a digital sequence of P-bit digital values that defines the temporal duration of the pixel drive signal and includes a first P-bit word that represents at least part of the digital input value at a location temporally corresponding to the duty cycle of the pixel drive signal as defined by the at least part of the digital input value. The comparator is connected to receive the digital sequence from the digital sequence generator and a second P-bit word from the memory. The second P-bit word constitutes at least part of the N-bit word. The comparator includes an output that provides the pixel drive signal and that changes state in response to correspondence between the first P-bit word and the second P-bit word.
The invention also provides a method for generating a pixel drive signal in response to an M-bit digital input value defining the apparent brightness of the pixel. The pixel drive signal has a duty cycle that sets the apparent brightness of the pixel. In the method, an N-bit word representing the digital input value is received and stored. A digital sequence composed of P-bit digital values is generated. The digital sequence defines the temporal duration of the pixel drive signal, and includes a first P-bit word that represents at least part of the digital input value at a location temporally corresponding to the duty cycle of the pixel drive signal as defined by the at least part of the digital input value. A second P-bit word constituting at least part of the stored N-bit word is compared with the digital sequence to generate the pixel drive signal. The pixel drive signal changes state in response to correspondence between the second P-bit word and the first P-bit word.
The different embodiments of the pixel driver and pixel drive signal generating method according to the invention are capable of driving an electrode applied to an electro-optical element to set the pixel to an apparent brightness in a monochrome display element and to set the pixel to an apparent brightness for each of two or more color components in a color display element. In the monochrome display element, the M-bit digital input value defines the apparent brightness of the pixel. In the color display element, the M-bit digital input value defines the apparent brightness of the pixel for the two or more color components, and the portions of the M-bit digital input value that define the apparent brightness for each of the color components may be received sequentially or simultaneously.
The pixel driver and pixel drive signal generating method according to the invention are also capable of driving an electrode applied to an electro-optical element to set the pixel to an apparent brightness in a monochrome display element and to set the pixel to an apparent brightness for each of two or more color components in a color display element in response to an N-bit palette code that represents the M-bit digital input value. The N-bit palette code is generated in response to the M-bit digital input value and identifies an element of a palette to represent the digital input value. The palette is composed of elements constituting a subset of a range of apparent brightnesses defined by digital input values having M bits. The palette is defined by a palette code table in which each of the elements is represented by an N-bit palette code and is defined by an M-bit value. In different embodiments of the pixel driver and the pixel drive signal generating method according to the invention, the N-bit palette code represents the apparent brightness of the pixel in the monochrome display element. In the color display element, the N-bit palette code may represent the apparent brightness of the pixel for one color component or may represent the apparent brightness of the pixel for each of two or more color components.
When the digital input value is represented by an N -bit palette code, the N-bit palette code is received and stored as the an N-bit word representing the digital input value, and the digital sequence is generated in response to the palette code table. The digital sequence includes the N-bit palette code for each of the elements of the palette at the location temporally corresponding to the duty cycle of the pixel drive signal defined by the M-bit value of the element.
The portion of the digital pixel driver according to the invention located in the pixel is simple and therefore enables the pixel to be small in size. This permits the pixel driver according to the invention to be incorporated into high-density display element. The portion of the pixel driver according to the invention located in the pixel is even simpler and smaller when the digital input value is represented by an N-bit palette code. This allows the pixel density to be further increased.
The digital pixel driver and the pixel drive signal generating method according to the invention generate a pixel drive signal having only one change of state per display period. This is in contrast to the above-mentioned conventional digital pixel drivers that generate bitwise time domain binary weighted sequences having many changes of state per display period. The pixel drive signals generated by the pixel driver and pixel drive signal generating method according to the invention therefore provide the same advantages in terms of the ferroelectric liquid crystal material bandwidth, buffer organization and link efficiency as the analog pixel drivers referred to above, but provide the additional advantage of operating directly in response to a digital input value.
When the pixel driver and pixel drive signal generating method set the apparent brightness of the pixel in a monochrome display, or set the apparent brightness of the pixel for each of two or more color components in response to all M bits of the digital input value, the pixel driver and pixel drive signal have the additional advantage of being able to operate in a low-power mode in which a new digital input value is received only when the digital input value changes.